The material properties of secondary-hardened carbon stainless steels are often limited by cementite precipitation during aging. Because the cementite is enriched with alloying elements, it becomes more difficult to fully dissolve the cementite as the alloying content of elements such as chromium increases. Undissolved cementite in the steel can limit toughness, reduce strength by gettering carbon, and act as corrosion pitting sites.
Cementite precipitation could be substantially suppressed in stainless steels by substituting nitrogen for carbon. There are generally two ways of using nitrogen in stainless steels for strengthening: (1) solution-strengthening followed by cold work; or (2) precipitation strengthening. Cold worked alloys are not generally available in heavy cross-sections and are also not suitable for components requiring intricate machining. Therefore, precipitation strengthening is often preferred to cold work. Precipitation strengthening is typically most effective when two criteria are met: (1) a large solubility temperature gradient in order to precipitate significant phase fraction during lower-temperature aging after a higher-temperature solution treatment, and (2) a fine-scale dispersion achieved by precipitates with lattice coherency to the matrix.
These two criteria are difficult to meet in conventional nitride-strengthened martensitic steels. The solubility of nitrogen is very low in the high-temperature bcc-ferrite matrix. And in austenitic steels, nitrides such as M2N are not coherent with the fcc matrix. Thus, there has developed a need for a martensitic steel strengthened by nitride precipitates.
Ideally, such steels will be corrosion resistant and exhibit high case hardness accompanied by excellent core properties including tensile yield strength above 150 ksi, tensile ultimate strength above 190 ksi, high fracture toughness and good elongation properties.